Existing redox flow batteries (RFB) are electrochemical energy stores. The compounds required for establishing the potential at the electrodes are dissolved, redox-active species which are converted into their other redox state in an electrochemical reactor during the charging or discharging process. For this purpose, the electrolyte solutions (catholyte, anolyte) are taken from a tank and actively pumped to the electrodes. Anode space and cathode space are separated in the reactor by means of an ion-selective membrane which usually has a high selectivity for protons. As long as electrolyte solution is pumped, power can be taken off. The charging process is then simply the reverse of this process. The quantity of energy which can be stored in an RFB is therefore directly proportional to the size of the storage tank. The power which can be taken off, on the other hand, is a function of the size of the electrochemical reactor.
RFBs have a complex system technology (BoP—Balance of Plant) which corresponds approximately to that of a fuel cell. Customary construction sizes of the individual reactors are in the range from about 2 to 50 kW. The reactors can be combined very simply in a modular fashion, and the tank size can likewise be adapted virtually at will. RFBs which operate using vanadium compounds as redox pair on both sides (VRFB) are of particular importance here. This system was described for the first time in 1986 (AU 575247 B) and is at present the technical standard. Further inorganic, low molecular weight redox pairs have been studied, including ones based on cerium (B. Fang, S. Iwasa, Y. Wei, T. Arai, M. Kumagai: “A study of the Ce(III)/Ce(IV) redox couple for redox flow battery application”, Electrochimica Acta 47, 2002, 3971-3976), ruthenium (M. H. Chakrabarti, E. Pelham, L. Roberts, C. Bae, M. Saleem: “Ruthenium based redox flow battery for solar energy storage”, Energy Conv. Manag. 52, 2011, 2501-2508), chromium (C-H. Bae, E. P. L. Roberts, R. A. W. Dryfe: “Chromium redox couples for application to redox flow batteries”, Electrochimica Acta 48, 2002, 279-87), uranium (T. Yamamura, Y. Shiokawa, H. Yamana, H. Moriyama: “Electrochemical investigation of uranium β-diketonates for all-uranium redox flow battery”, Electrochimica Acta 48, 2002, 43-50), manganese (F. Xue, Y. Wang, W. Hong Wang, X. Wang: “Investigation on the electrode process of the Mn(II)/Mn(III) couple in redox flow battery”, Electrochimica Acta 53, 2008, 6636-6642) and iron (Y. Xu, Y. Wen, J. Cheng, G. Cao, Y. Yang: “A study of iron in aqueous solutions for redox flow battery application”, Electrochimica Acta 55, 2010, 715-720). However, these systems are based on metal-containing electrolytes which are toxic or damaging to the environment.
VRFB reactors can at present be obtained in blocks of from 1 to 20 kW. Higher power outputs are achieved by modular connection of these. Each individual block contains a plurality of planar cells which are connected in series to achieve a higher voltage. This bipolar construction largely corresponds to the construction of a PEM fuel cell. A perfluorinated polymer having sulfonic acid groups, usually DuPont Nafion® 117, is utilized as membrane. Other polymers have been described, for example polymers based on SPEEK (Q. Luo, H. Zhang, J. Chen, D. You, C. Sun, Y. Zhang: “Nafion/SPEEK composite: Preparation and characterization of Nafion/SPEEK layered composite membrane and its application in vanadium redox flow battery”, J. Memb. Sci. 325, 2008, 553-558), PVDF (J. Qiu, J. Zhang, J. Chen, J. Peng, L. Xu, M. Zhai, J. Li, G. Wei: “Amphoteric ion exchange membrane synthesized by radiation-induced graft copolymerization of styrene and dimethylaminoethyl methacrylate into PVDF film for vanadium redox flow battery applications”, J. Memb. Sci. 334, 2009, 9-15), QPPEK (S. Zhang, C. Yin, D. Xing, D. Yang, X. Jian: “Preparation of chloromethylated/quaternized poly(phthalazinone ether ketone) anion exchange membrane materials for vanadium redox flow battery applications”, J. Memb. Sci. 363, 2010, 243-249), fluorine-free sulfonated polyarylene (D. Chen, S. Wang, M. Xiao, Y. Meng: “Synthesis and properties of novel sulfonated poly(arylene ether sulfone) ionomers for vanadium redox flow battery”, Energy Conv. Manag. 51, 2010, 2816-2824) or inorganic-organic composite materials comprising SiO2 (J. Xi, Z. Wu, X. Qiu, L. Chen: “Nafion/SiO2 hybrid membrane for vanadium redox flow battery”, J. Pow. Sour. 166, 2007, 531-536), but, in contrast to Nafion membranes, are not yet practical and commercially available. The same applies to nanofiltration membranes which allow the protons of the acid electrolyte to pass through and hold back the vanadium salts (Hongzhang Zhang, Huamin Zhang, Xianfeng Li, Zhensheng Mai, Jianlu Zhang: “Nanofiltration (NF) membranes: the next generation separators for all vanadium redox flow batteries (VRBs)”, Energy & Environmental Science, 2011, 4, 1676-1679). Regardless of these, the same disadvantages such as high cost and environmental pollution in the case of a major accident and also short life of the cells would also apply here.
In the present state of the art, the use of ion-conducting membranes limits further commercialization since standard Nafion® membranes are expensive, fluorine-containing, mechanically weak; furthermore, these swell to a great degree and are susceptible to an electrochemical short circuit due to inward diffusion of vanadium ions.
Purely organic redox compounds have hitherto been used very little in RFBs. Thus low molecular weight 2,2,6,6-tetramethylpiperidinyloxy (TEMPO) and N-methylphthalimide have been used in an RFB having an ion-conducting membrane (Z. Li, S. Li, S. Q. Liu, K. L. Huang, D. Fang, F. C. Wang, S. Peng: “Electrochemical properties of an all-organic redox flow battery using 2,2,6,6-tetramethyl-1-piperidinyloxy and N-methylphthalimide”, Electrochem. Solid State Lett. 14, 2011, A171-A173). Furthermore, rubrene is ruled out because of high costs and very low solubility, despite good electrochemical properties (cf. H. Charkrabarthi, R. A. W. Dryfe, E. P. L. Roberts, Jour. Chem. Soc. Pak. 2007, 29, 294-300 “Organic Electrolytes for Redox Flow Batteries”).
Batteries based on 2,3,6-trimethylquinoxaline also utilize expensive ion-selective Nafion® membranes (F. R. Brushett, J. T. Vaughey, A. N. Jansen: “An All-Organic Non-aqueous Lithium-Ion Redox Flow Battery”, Adv. Energy Mater. 2012, 2, 1390-1396).
Pyrazine-based cyanoazacarbones (U.S. Pat. No. 8,080,327 B1) have been used both as anolyte and as catholyte, with ion-conducting membranes based on cation exchangers and anion exchangers being used for separating the electrode spaces. These membranes are expensive and in each case permeable only to a particular class of ions. This is reflected, in particular, in a disadvantageous system construction which has to utilize an electrolyte reservoir between the anolyte circuit and the catholyte circuit. This is necessary in order to ensure charge equalization/mixing of the anions which diffuse through the anion exchanger membrane into the reservoir and the cations which diffuse through the cation exchanger membrane into the reservoir.
Apart from the organic redox compounds, low molecular weight metal-organic compounds are described (M. H. Chakrabartia, R. A. W. Dryfe, E. P. L. Roberts: “Evaluation of electrolytes for redox flow battery applications”, Electrochimica Acta, 52, 2007, 2189-2195). Here, organic ligands which complex inorganic metal salts are used. Such ligands are, for example, bipyridyl, terpyridyl, phenanthroline or imidazoles (US 2012/0171541 A1). For these systems, too, expensive ion-conducting membranes such as Nafion® or amine-functionalized polystyrene derivatives have to be used. The same applies to redox flow batteries based on low molecular weight ruthenium-bipyridine complexes which, for example, utilize the anion exchanger membranes Neocepta®. Other membranes are, in contrast, permeable to these complexes and lead to a low efficiency of the battery in this case (Y. Matsuda, K. Tanaka, M. Okada, Y. Takasu, M. Morita, T. Matsumura-Inoue: “A rechargeable redox battery utilizing ruthenium complexes with non-aqueous organic electrolyte”, J. Applied Electrochem. 18, 1988, 909-914).
It is an object of the invention to provide, by use of new materials and membranes and with very little outlay, an inexpensive and long-lived redox flow cell which even in the event of a possible serious accident brings about little environmental pollution by its redox-active compounds.